348 PROF. J. W. JUDD ON THE PROPYLITES 



2'7, and those in which the limits are 2*7 and 2-9. As a general 

 rnle (though colour is often a very unsafe guide) the former group . 

 comprises pale-coloured rocks, while the latter are generally dark- 

 coloured. 



The less dense, pale-coloured rocks, we shall presenth- see, were 

 originally hornblende- and mica-andesites, and the heavier dark- 

 coloured rocks are, for the most part, pyroxene-andesites. 



Looking more closely into the results, we find that differences in 

 specific gravity in each of these classes of rocks is dependent j^rs^, 

 on the degree of development of crystallization in them, and secondly 

 on the amount of chemical alteration which they have undergone. 



As an example of difference of density in the more acid t3^pes, I may 

 cite the case of the hornhlende-propylite of Beinn Talaidh (Beinn 

 Talla) in Mull. In the deep Corry of Tomsleibe the rock is highly 

 crystalline, and, indeed, approaches a diorite, having a specific 

 gravity of 2-68 ; while in the upper and superficial portions of the 

 mass the rock is compact, and has a density of 2-60. Glassy 

 varieties of the amphibolic rocks have a density of only a little over 

 2-4. 



Among the more basic types, I may cite the pyroxene-propylites 

 of Mingary Castle in Ardnamurchan. Highly cr5-stalline forms of 

 this rock have a density of 2'88, while the specific gravity of the 

 compact varieties of the same rock is only 2* 75. At Bealach a' 

 Mhaim, in Glen Brittle, we find a rock of this type with a specific 

 gravity of 2*89, which passes locally into a glass with a density of 

 only 2-G3. 



The effect of the processes of alteration upon these rocks is 

 general^ to lower their specific gravity. Thus the much altered 

 hornblende-propylites, with abundant chlorite and epidote developed 

 in their mass,, are found to have a density of 2'b, or even less, while 

 the similarly altered pyroxene-propylites have a density which 

 seldom much exceeds, and sometimes does not reach 2-7. 



In cases where it is not practicable to make a complete chemical 

 or microscopical study of the rocks, a determination of specific 

 gravity affords a much safer criterion for their discrimination than 

 colour. Some of the hornblende-propylites are of a very dark grey, 

 and indeed almost black colour, though seldom exhibiting the lustrous 

 jet-black of the olivine basalts ; while, on the other hand, extreme 

 alteration may sometimes cause the pyroxene-propylites to assume 

 a pale grey and almost white colour. 



The distinction of the Scottish propylitcs into two groups is boiTie 

 out when we examine their chemical composition. A. number of 

 chemical analyses have been made for me by Mr. T. H. Holland in 

 the Geological llcsearch Laboratory of the Normal School of Science, 

 and for some silica determinations I am indebted to Mr. Grenville 

 Cole, F.G.S., and Mr. J. H. Power. ] am also under great obliga- 

 tions to my colleague Professor Thorpe, F.li.S., for allowing a number 

 of analyses of tlicse interesting rocks to be carried out under his 

 direction in the chemical laboratories of the same Institution. Tlicse 

 analyses enable us to make comparisons between the different types 



